Spot welding has been used for many years in industry, in preference to riveting, for joining sheet metal because of its low cost. However, although spot welding has largely supplanted riveting for many applications, engineers have been reluctant to use spot welding on highly stressed components, (as on airplanes, for example), because of the relatively low fatigue strength of spot welds compared to riveting.
In recent years, it has been found that when metals are joined by spot welding but the joint is also reinforced with a structural adhesive, it develops a fatigue strength significantly greater than a riveted joint. This method of joining metal members, such as aluminum alloy, is called "weld bonding" or "adhesive-spot-welding." This method has generally included the steps of cleaning and etching the surfaces and applying an adhesive upon the faying surfaces to be joined and then spot welding through the adhesive. Another approach of the method has been to clean and etch the surfaces, spot weld, and then apply the adhesive along the edges of the joint, allowing the adhesive to flow into the joint interface by capillary action. After either approach above, the adhesive in the joint is cured by placing the joined metals in an oven for a suitable heat cycle. Since weld-bonding is more economical than riveting, and its fatigue strength is greater, engineers hereafter will probably specify its use in lieu of riveting on highly stressed parts such as aircraft fuselages.
In order to obtain successful usage of weld-bonding in industry, it is generally recognized that weld bonded joints should be characterized by:
1. Consistently high adhesive strength, and PA1 2. Consistently high strength spot welds, and PA1 3. Consistent durability in a variety of corrosive environments. PA1 1. Provide the optimum surface preparation cleaning and etching procedure for optimum strength joints and corrosion resistance for adhesive metal bonding. PA1 2. If necessary, apply and cure a corrosion resistant primer to the surfaces prepared by step 1 above. PA1 3. Abrade small matching circular areas upon the faying surfaces, thereby removing the primer and the chemical residues remaining from the surface preparation in step 1. PA1 4. Spot weld the parts together at the abraded areas using a suitable template. PA1 5. Introduce a high strength structural adhesive with low viscosity on one edge of the welded parts and PA1 6. Provide a vacuum to assist the capillary flow of the adhesive through the joint until it becomes visible at the opposite edge to assure complete coverage. PA1 7. If necessary, cure the weld-bonded assembly using a suitable heat cycle. PA1 1A. Apply masking discs upon the surfaces (between steps 1 and 2 above). PA1 2B. Remove the masking discs from the surfaces (between steps 2 and 3 above).
The problem in the past was to get a surface preparation which was compatible with obtaining all these elements. For example, a sodium dichromatic/sulfuric acid etch has been used to prepare aluminum surfaces for adhesive bonding. However, this treatment, which provides excellent adhesive bonded joint strengths, has been shown to degrade seriously the quality and uniformity of the spot weld. On the other hand, the use of a spot-welding cleaning etch which provided high quality welds, also provides significantly lower adhesive bonded joint strengths and durability.
Better and more consistent spot welds are obtained in the procedural approach above where the spot welding is done first on bare metal and the adhesive is applied after the spot welding. This is because optimum spot welds require high cleanliness without contaminants and frequently the adhesive constitutes a contaminant when using the weld through approach. But when the spot welding is done first and the adhesive flow-through the joint depends on capillary action alone, the adhesive coverage over the entire joint area is often incomplete and frequently contains many areas devoid of adhesive.
Finally, use of a corrosion resistant primer cannot be used readily in weld-bonding with prior methods of surface preparation because the primer must be cured prior to any subsequent processing, and the cured primer acts as an electrical insulator which precludes the formation of acceptable spot welds.